1. The Field of the Invention
The present invention relates generally to x-ray systems, devices, and related components. More particularly, exemplary embodiments of the invention concern systems, methods and devices for implementing automatic control of Z axis focal spot location.
2. The Relevant Technology
The ability to consistently develop high quality radiographic images is an important element in the usefulness and effectiveness of x-ray devices as diagnostic tools. However, various factors relating to the construction and/or operation of the x-ray device often serve to materially compromise the quality of radiographic images generated by the device. Such factors include, among others, vibration caused by moving parts of the x-ray device, and various thermally induced effects such as the occurrence of physical changes in the x-ray device components as a result of high operating temperatures and/or thermal gradients.
The physical changes that occur in the x-ray device components as a result of the relatively high operating temperatures typically experienced by the x-ray device are of particular concern. Not only do the high operating temperatures impose significant mechanical stress and strain on the x-ray device components, but the heat transfer effected as a result of those operating temperatures can cause the components to deform, either plastically or elastically.
While plastic deformation of an x-ray device component is a concern because it may be symptomatic of an impending failure of the component, elastic deformation of the x-ray device components under high heat conditions is problematic as well. For example, as the various components and mechanical joints are subjected to repeated elastic deformation under the influence of thermal cycles, the connections between the components can loosen and the components may become misaligned or separated.
In addition, the elastic deformation of x-ray device components has significant implications with respect to the performance of the x-ray device. One area of particular concern relates to the effects of the elastic deformation of x-ray device components on focal spot location and positioning. As discussed below, the quality of the radiographic images produced by the device depends largely on reliable and consistent positioning of the focal spot, any changes to the location and positioning of the focal spot during the generation of the radiographic image act to materially impair the quality of the image and, thus, the effectiveness of the x-ray device.
In general, the generation of a radiographic image involves the use of a cathode, or other electron emitter, to direct a beam of electrons at an anode, or target, having a target surface composed of a material such that, when the target surface is struck by the electrons, x-rays are produced. In order to produce a high quality image, the electrons of the electron beam are focused at a particular location, or focal spot, on the surface of the target.
As suggested above, problems occur when the location of the focal spot changes. The focal spot location can change in various ways. In some cases, the focal spot may shift within the imaginary X-Y plane that is generally perpendicular to the beam of electrons. So long as the focal spot remains at a desired Z axis position with respect to the detector however, such X-Y plane shifts may not be cause for particular concern. However, a shift in the Z axis location of the focal spot, as often occurs in connection with elastic deformation of x-ray device components such as the anode assembly and housing, is much more problematic.
With regard to the foregoing, the Z axis refers to an imaginary axis along which the emitted electrons travel from the cathode to the target surface of the anode. Thus, the Z axis is perpendicular to the X-Y plane. The focal spot is susceptible to movement along the Z axis as a result of relative changes in the positioning of the cathode with respect to the target surface of the anode. One of the most prevalent causes of such changes to the location of the focal spot is thermally induced elastic deformation of the anode assembly and/or x-ray device housing.
Typically, the anode assembly experiences a thermally induced deformation that causes the anode assembly to expand along the Z axis toward the cathode, thereby decreasing the distance between the cathode and the target surface, and effectively moving the focal spot from its intended position relative to the detector. However, elastic deformation of other x-ray device components may likewise cause Z axis focal spot motion. In any case, Z axis movement of the focal spot materially impairs the quality of the radiographic image.
A variety of attempts have been made to resolve the problem of thermally induced Z axis motion of the focal spot. As discussed below however, such attempts have proven ineffective and/or undesirable, for a variety of different reasons.
One general approach to the problem of Z axis focal spot motion concerns the use of electro-mechanical systems and devices to physically move the x-ray tube unit in order to compensate for thermally induced focal spot motion. In theory, the motion of the x-ray tube unit should offset any motion of the anode assembly, for example, so that the net change in the position of the focal spot is minimized. This particular approach has proven problematic in practice however.
For example, such electro-mechanical systems are typically quite complex and, accordingly, add significantly to the overall expense of the associated x-ray device. A related problem is that initial installation and testing of the system is often a lengthy and expensive process. Further, because these electro-mechanical systems introduce a variety of additional components and, thus, increase the number of potential failure points, such systems tend to reduce the overall reliability of the x-ray device. In a related vein, such electro-mechanical systems are typically maintenance intensive and must be frequently monitored in order to ensure proper functioning.
Yet another approach employed in an attempt to resolve the problem of Z axis focal spot motion involves the use of a software algorithm that gathers focal spot position data at various temperatures and uses the gathered information to determine an optimal distance between the cathode and anode assembly. More particularly, radiographic images are generated over temperatures ranging from a “cold” tube condition, or ambient temperature, to a “hot” tube condition, or anticipated steady state operating temperature. At each different temperature in the range, the location of the focal spot is determined. The gathered information can then be used to determine the focal distance at which the best radiographic image is produced. The cold positions of the cathode and/or anode assembly is/are then adjusted such that the ideal focal distance will be achieved at normal x-ray tube operating temperatures.
A significant disadvantage with this approach however, is that the x-ray device cannot be used “out of the box” to generate radiographic images. Rather, significant setup time and testing are required before the optimal focal spot location can be determined and image generation can begin. Such setup time and testing increase the overall expense associated with operation of the x-ray device.
Further, such an approach lacks a suitable feedback and/or compensation mechanism. In particular, the focal spot location data that is gathered concerning the x-ray tube is based on a like-new condition of the x-ray device and, accordingly, fails to provide any compensation for Z axis focal spot location changes that may occur during the break-in period of the device and/or focal spot location changes that typically occur as the x-ray device ages. Thus, a gradual, and sometimes undetected, degradation to the radiographic images can occur over time and, while the incremental change in the quality of the images may be subtle, such changes may seriously impair the diagnostic value of those images.
As the foregoing suggests, the x-ray device will require modification, at some point, to compensate for age related, and other, effects that have occurred since the x-ray device was initially placed into service. This modification is performed in the same fashion as at initial setup of the device and, depending upon the age and condition of the device, may be required to be performed several times over the life of the x-ray device, thereby increasing downtime as well as the overall cost of operating the device.
Finally, another approach to the problem of Z axis focal spot motion involves a passive compensation mechanism. More particularly, this approach involves attempting to compensate for anticipated Z axis focal spot motion by designing the x-ray device and associated components in such a way that the net thermally induced motion of the focal spot is minimized. This attempt to passively resolve the problem of Z axis focal spot motion has proven problematic in practice however.
For example, it is often difficult to design engineering models that can accurately predict the various thermally induced effects that will occur in the numerous components that make up the x-ray device. Moreover, the failure to account for all relevant variables and/or the failure to accurately model such variables seriously impairs the usefulness of the results obtained in connection with the engineering model. Thus, significant study, engineering analysis, and trial and error testing may be required before any useful conclusions can be drawn as to the nature of the structures that must be employed to minimize Z axis focal spot drift at operating temperatures.
Another problem with the aforementioned passive compensation approach is that even if a suitable engineering model is developed, the construction and assembly of the x-ray device structures required to ensure minimal focal spot drift can be quite expensive. As well, the physical and dimensional requirements of some x-ray devices are simply inconsistent with the use of the structures that the engineering model indicates are necessary for focal spot movement compensation.
Moreover, an x-ray device constructed in accordance with such engineering models will likely experience Z axis focal spot drift at some point during its lifespan. This is due in part to the fact that the model is typically based on the characteristics of a new x-ray device and does not include any mechanism to compensate for Z axis focal spot drift that results from physical changes that occur to the x-ray device as the device ages.
A further operational problem with the passive compensation approach relates to the response of the x-ray device when subjected to operating temperatures. In particular, the location of the focal spot tends to oscillate sinusoidally with respect to the reference point, or desired focal spot location, before the system stabilizes at the desired location.
Further, there may be some hysteresis reflected in the response of the x-ray device such that a time lag can occur between a change in operating temperature, and the corresponding shift in the focal spot location. In other cases, the hysteresis may be reflected by a failure of the x-ray device to fully reestablish the desired focal spot location after occurrence of a change in operating conditions. In any event, slow and/or incomplete responses to changes in operating conditions result in undesirable Z axis focal spot positioning.
In view of the foregoing, and other, problems in the art, it would be useful to provide relatively low cost systems, methods and devices that automatically control Z axis focal spot location in a wide variety of operating conditions.